Microfabricated structures and processes for manufacturing same

ABSTRACT

Various techniques for the fabrication of highly accurate master molds with precisely defined microstructures for use in plastic replication using injection molding, hot embossing, or casting techniques are disclosed herein. Three different fabrication processes used for master mold fabrication are disclosed wherein one of the processes is a combination of the other two processes. In an embodiment of the first process, a two-step electroplating approach is used wherein one of the metals forms the microstructures and the second metal is used as a sacrificial support layer. Following electroplating, the exact height of the microstructures is defined using a chemical mechanical polishing process. In an embodiment of the second process, a modified electroforming process is used for master mold fabrication. The specific modifications include the use of Nickel-Iron (80:20) as a structural component of the master mold, and the use of a higher saccharin concentration in the electroplating bath to reduce tensile stress during plating and electroforming on the top as well as sides of the dummy substrate to prevent peel off of the electroform. The electroforming process is also well suited towards the fabrication of microstructures with non-rectangular cross sectional profiles. Also disclosed is an embodiment of a simple fabrication process using direct deposition of a curable liquid molding material combined with the electroforming process. Finally, an embodiment of a third fabrication process combines the meritorious features of the first two approaches and is used to fabricate a master mold using a combination of the two-step electroplating plus chemical mechanical polishing approach and the electroforming approach to fabricate highly accurate master molds with precisely defined microstructures. The microstructures are an integral part of the master mold and hence the master mold is more robust and well suited for high volume production of plastic MEMS devices through replication techniques such as injection molding.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application claims priority to provisional U.S. Patent ApplicationsSer. Nos. 60/506,641; 60/506,226; 60/506,321; 60/506,424; and 60/506,635all filed on Sep. 26, 2003, and all of which are incorporated herein byreference in their entirety.

GOVERNMENT SUPPORT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of grant no. AFF30602-00-1-0569 awarded by the Defense Advanced Research ProjectsAgency (DARPA).

This patent application is being filed concurrently with U.S. PatentApplications having attorney docket numbers 200057.00008, 200057.00009,200057.00010, and 200057.00011, which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

Embodiments of the present invention generally relate to the fabricationof ultra-high precision master molds for high volume production ofmicrofabricated structures through plastic replication processes likeinjection molding and hot embossing. Herein is described an approach forfabricating microstructures with extremely uniform features and highsurface quality using a two-step electroplating process followed by aplanarization approach where one of the metals is used as a sacrificialsupport structure. This technique is particularly well suited forfabricating rectangular cross-section microstructures as used inmicrofluidic devices. Nickel Iron (80:20) alloy is introduced as a lowstress material for the fabrication of master molds using the process ofelectroforming. The electroforming process can be used for fabricatingmicrostructures with rectangular cross-sections suited towardsmicrofluidic applications, as well as other shapes more suited foroptical applications. Also, a very simple technique for manufacturing ahigh density microlens array for optical applications is describedherein.

BACKGROUND OF THE INVENTION

MEMS (Micro Electro Mechanical Systems) technology has enabledfabrication of miniaturized devices with applications in a variety offields like aerospace, medicine and telecommunications. Traditionallymost MEMS based devices have been fabricated on Silicon or Glasssubstrate using the same technology as developed for themicroelectronics industry. An example of this is disclosed by Chow et alin U.S. Pat. No. 6,167,910 (incorporated herein in its entirety byreference), wherein a three-dimensional microfluidic device isfabricated by stacking glass/Silicon substrates onto which microchannelsfor flow confinement have been defined. However, specifically formicrofluidic and more specifically for BioMEMS related applications,Silicon and Glass were observed to have many limitations in terms of thesurface exposed to the fluids and/or biological samples. One specificproblem is non-specific adsorption of proteins commonly encountered inbiological samples. Considerable care and conditioning are required touse Silicon or glass substrates in such applications. Furthermore,Silicon and Glass are processed using the so-called “serial process”wherein, for some of the processing steps, each substrate is processedindividually (even if it forms part of a batch). With the high intrinsiccost of these substrates, the serial processing further adds to the costof the device.

Hence, there has been considerable interest in using polymer or plasticsubstrates for MEMS and BioMEMS applications. The appropriate plasticsoffer numerous advantages such as low-cost, high durability, highstrength, and excellent biocompatibility characteristics. Another areawhere plastic devices are obviously suited is Optical MEMS whereinoptical transmission characteristics with respect to a range ofwavelengths are critical. Polymer/Plastic substrates can be broadlydivided into two categories: thermoplastic materials, which deform whenheated, and thermoset materials, which solidify after heating. Amongstthe two, thermoplastics have gained wider acceptance owing to theability of using mass-manufacturing techniques such as injection moldingand embossing. A notable exception to this is the use ofPoly-dimethylsiloxane (PDMS), which is a thermoset material forrapid-prototyping of microfluidic devices as described in U.S. Pat. No.6,686,184 and WO02100542A1, incorporated herein in their entirety byreference.

The most common approach to fabricate plastic microfluidic devices is tocreate a “master mold” and a suitable replication technique such as hotembossing or injection molding to transfer the pattern to athermoplastic substrate. In earlier approaches, again Silicon was usedas the material for the master mold and the desired features were formedby either wet-chemical etching or deep reactive ion etching (DRIE) asdescribed in U.S. Pat. No. 6,136,243 and as discussed by G. Kovacs,Micromachined Transducers Sourcebook, WCB-McGraw Hill, New York, 1998.However, Silicon master-mold cannot be easily used with injectionmolding equipment due to processing complexities and the relativelyfragile nature of the Silicon substrate. Silicon substrates are mostcommonly limited to replication with hot-embossing techniques asdescribed in WO4022302A3 (incorporated herein in its entirety byreference). Hot embossing is an inherently slow-process and cannot matchthe short-cycle times of injection molding.

The most commonly accepted substrate material for injection molding issome form of a metallic substrate. Metallic substrates offer the desiredstrength and machining characteristics required for injection moldingequipment. A common technique for creating master molds is the use ofhigh-precision milling tools to define precise features onto a metalsubstrate. However, the milling process is limited in terms ofachievable feature size and aspect ratio (the ratio of height to widthfor a microstructure). UV-LIGA is another one of the commonly usedtechniques to form micro fabricated master molds. It involves creating amold from a photoresist, deposited onto a substrate, usingphotolithography followed by electroplating within the photoresist mold.After electroplating, the photoresist mold is dissolved using suitablesolvents and the electroplated metal pattern (usually of the samematerial as the substrate) acts as a master mold for plasticreplication.

One of the difficulties in this method is the fabrication of structuresthat do not have a rectangular cross-section. One example of such astructure being the hemispherical shape required to fabricated lenses oran array of microlenses for optical MEMS applications. A possiblesolution to this problem is offered by the use of the so-called“gray-scale” lithography techniques, wherein different sections of thephotoresist are exposed to continuously varying UV energy therebyallowing for the formation of “sloped” or “rounded” features asdescribed in U.S. Pat. No. 6,410,213 (incorporated herein in itsentirety by reference).

Another major drawback in UV-LIGA based master mold fabrication isnon-uniform electroplating thickness in photoresist mold patterns havingdifferent dimensions (specifically different widths). Due to the processdescribed as “current-crowding”, the plating rate is faster in channelswith smaller widths than in channels with higher widths due to theconcentrated current flux in narrow channels. This may give rise toproblems where the microstructure needs to be very accurately definede.g. in microfluidic applications. Furthermore, another problem of thecurrent-crowding effect is the non-uniform cross-sectional profileachieved after electroplating, wherein typically (along thecross-section) the center of the microstructure is plated to a lowerheight as compared to the edges of the microstructure. This problem isdiscussed clearly in JP1165794A2 (incorporated herein in its entirety byreference), wherein a sharpened anode was used to plate within a narrowfeature, and pulled out at approximately the same rate as the platingoccurred. Though, this approach allows for uniform plating, it is fairlycomplex to set it up and may not be suitable for plating large areas.

Another method for creating master molds is by using the techniques ofelectroforming. In this approach, a photoresist mold is created on adummy substrate. Alternately, the mold pattern can also be directlyetched into the dummy substrate using wet chemical etching,electro-discharge machining (EDM) or conventional micro-milling.Following this, a metal seed layer is deposited on the photoresist (orsubstrate) mold and an appropriate metal is electroplated beyond thethickness of the mold. In most cases (for microfabricated master molds),the electroformed metal is plated to a thickness of <1 mm and thenpeeled off the dummy substrate. The concept of electroforming iscertainly not new as illustrated by applications (JP54067561A2) datingback to 1979. The process of electroforming is well known to thoseskilled in the art and extensively described in WO03041934A1 andJP62111755A2, incorporated herein in their entirety by reference,amongst others.

One of the primary issues in electroforming is stress control. When ametal is electroplated to large thicknesses, the tensile and compressivestresses dominate the final electroform shape. If the stress is notcontrolled properly, the electroformed master mold is severely distortedmaking it useless for plastic replication. This issue has limited theuse of electroforming techniques for injection molding applications.

On the other hand, electroforming techniques offer the advantage ofnon-rectangular cross-sections that are suitable for plastic replicationtechniques. Specifically, if a master mold feature is wider at the topthan the bottom, it is not possible to injection mold (or emboss) a copyof it since the mold pattern will be “stuck” into the plastic substrate.However, when the master mold pattern is narrower at the top, it iseasier to separate the master mold and the replicated plastic part. Suchprofiles are very difficult to create (except by using complexgray-scale lithography techniques as explained earlier) using UV-LIGAtechniques. An innovative approach is described in JP2297409A2(incorporated herein in its entirety by reference), wherein thephotoresist mold is heated beyond the heat resistance temperature of thephotoresist thereby causing reflow of the photoresist, leading totapered structures, which are narrower at the top. Electroforming isdone over the reflowed photoresist mold to create a master moldespecially designed for easy separation of the master mold fromreplicated plastic.

For Optical MEMS applications, an array of recessed semi-sphericalcavities within the master mold can be used to replicate Piano-Convexmicrolens arrays onto the replicated plastic. Several innovativeapproaches have been used to create this pattern in a master molddesigned for electroforming. For example, in WO03041934A1 andJP1212789A2 (incorporated herein in their entirety by reference), thedummy substrate is directly machined to form suitable shapes and thenthe master mold is electroformed over it. U.S. Pat. No. 5,705,256(incorporated herein in its entirety by reference), discloses atechnique wherein, the recessed cavities are defined by isotropic, wetchemical etching- and subsequently used for electroforming of the mastermold and yet another approach is described in U.S. Pat. No. 6,436,265(incorporated herein in its entirety by reference). JP2001290006A2(incorporated herein in its entirety by reference), discloses a methodwherein the microlenses are formed directly onto a flat plasticsubstrate by depositing a controlled amount of another plastic materialand heating the deposited plastic to its reflow temperature therebyforming the microlens array.

Finally, an issue of concern in master molds created using eitherUV-LIGA or electroforming techniques, is the surface roughness of themicrostructures. For most BioMEMS applications, increased surfaceroughness leads to poor performance; specifically in the case ofCapillary Electrophoresis (CE) chips, wherein poor surface quality canrender the chip unusable for separation applications. Obviously, forOptical MEMS applications, even slight surface imperfections can lead todeviation in the optical path characteristic ergo, poor deviceperformance.

For injection molding, the typical setup includes an arrangement toinject molten plastic material into a mold block. The mold blockcontains the features to be replicated. In most cases, the master moldis fabricated either as an integral part of the mold block or as acomponent, which can be assembled into the mold block. Obviously, whenmicrofabrication techniques, such as UV-LIGA, are used the master moldis always an independent piece that is assembled into the mold block asexplained in R. Trichur et al. in the Proceedings of the 6thInternational Conference on Micro Total Analysis Systems (micro-TAS2002), Nara, Japan, Nov. 3-7, 2002, pp. 560-562, and A. Puntambekar etal. in Proceedings of the 6th International Conference on Micro TotalAnalysis Systems (micro-TAS 2002), Nara, Japan, Nov. 3-7, 2002, pp.422-424 and J.-W. Choi et al. in Proceedings of the 5th InternationalConference on Micro Total Analysis Systems (micro-TAS 2001), Monterey,Calif., Oct. 21-25, 2001, pp. 411-412.

SUMMARY OF THE INVENTION

Certain embodiments of the present invention seek to address theshortcoming listed above to develop a master mold for plasticreplication with precisely defined microstructures and ultra-low surfaceroughness. Also, modifications to the electroforming process aredisclosed to make it more amenable towards fabrication of master moldsfor injection molding. Finally, a simple yet elegant approach ispresented to fabricate a master mold for microlens arrays on plasticsubstrates.

Disclosed herein is an embodiment of a two-step electroplatingtechnique, wherein the height of the microstructures on the master moldis controlled precisely using a polishing step after electroplating ofthe microstructures. The structural integrity of the microstructures ispreserved by the use of a second sacrificial support metal which is alsoelectroplated, following the first electroplating step. The secondsacrificial metal is selectively etched out after the polishing step.The polishing step ensures that all the microstructures are of uniformheight (across the entire substrate) and furthermore each microstructureis exhibits uniform height along its cross-sectional profile. Thepolishing step also ensures that the surface roughness of themicrostructures is minimized to yield high-quality replicated features.

Also disclosed herein are embodiments for making modifications to theelectroforming technique to make it suitable for master mold fabricationfor injection molding applications. More particularly, certainembodiments of the present invention use Nickel-Iron (80:20) alloy as alow stress material extremely suitable for electroforming where thestress can be controlled easily just by adjusting the current density,temperature, pH, and composition of the electroplating bath to obtainelectroforms that are suited as master molds for plastic replication.Previously, Nickel-Iron (80:20) electroplating has been used to developsoft magnetic material due to this material's excellent magneticproperties. This is the first application where the alloy is used as astructural component of the master mold. Other modifications to theelectroforming process will be apparent in the section entitled“Detailed Description of the Invention”.

Certain embodiments of the present invention provide an elegant solutionto creating a negative image of an array of Piano-Convex microlenses ona master mold and subsequently replicating the Plano-Convex lens arraystructure on the plastic substrate.

Embodiments of the present invention overcome the deficiencies andinadequacies in the prior art as described in the previous section andas generally known in the industry.

Certain embodiments of the present invention are concerned withdeveloping a master mold for plastic replication, wherein the mastermold is a discrete component, which can be easily assembled into theinjection mold block for plastic replication.

Certain embodiments of the present invention are concerned withdeveloping a master mold using a modified UV-LIGA fabrication process,specifically the two-step electroplating process with one of theelectroplating used for sacrificial metal deposition, to create mastermolds with microstructures of uniform height across the entire mastermold.

Other embodiments of the present invention are concerned with developinga master mold using a modified UV-LIGA fabrication process, specificallythe two-step electroplating process with one of the electroplating usedfor sacrificial metal deposition, to create master molds wherein, themicrostructures have uniform height across their cross-section.

Certain embodiments of the present invention are concerned withdeveloping a master mold using a modified UV-LIGA fabrication process,specifically the two-step electroplating process with one of theelectroplating used for sacrificial metal deposition, to create mastermolds with surface roughness less than 50 nanometers (nm).

Certain embodiments of the present invention are concerned withdeveloping a master mold suitable for replication of microfluidicstructures onto a plastic substrate.

Alternate materials have been investigated for the electroformingprocess thereby allowing for fabrication of very thick (more than 0.5mm) electroforms.

Certain embodiments of the present invention are concerned withdeveloping a fabrication process, using the alternate materials, forfabricating planar electroforms with minimal bending and/or curvaturealong the diameter of the master mold.

Certain embodiments of the present invention are concerned withdeveloping a master mold suitable for replication of a microlens arrayonto a plastic substrate.

Certain embodiments of the present invention are concerned withdeveloping a simplified process for the fabrication of the microlensarray on a plastic substrate by replication techniques.

Other embodiments, features and advantages of the present invention willbecome apparent from the detailed description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 b are schematic sketches illustrating the replaceable molddisk technology and also shows actual photographs of the mold block andmaster mold with microfabricated features, in accordance with variousembodiments of the present invention.

FIGS. 2 a-2 g are schematic sketches explaining the UV-LIGA processnormally used for master mold fabrication.

FIGS. 3 a-3 j are schematic sketches explaining the sequence of stepsused for fabricating the master mold using the two-step electroplatingtechnique, in accordance with an embodiment of the present invention.

FIGS. 4 a-4 i are schematic sketches explaining the sequence of stepsused for fabricating the master mold using the electroforming approach,in accordance with an embodiment of the present invention.

FIGS. 5 a-5 f are schematic sketches explaining the fabrication processfor the manufacture of the master mold and subsequently the plasticreplica with an array of microlenses, in accordance with an embodimentof the present invention.

FIGS. 6 a-6 k are schematic sketches showing the sequence of steps inthe modified electroforming process for the manufacture of a highlyaccurate and precise master mold, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Broadly stated, certain embodiments of the present invention provide twotechnologies intended for developing master molds for plasticreplication using injection molding techniques. The first part hereindiscloses an embodiment of a two-step electroplating/polishing approachwherein, one of the electroplated metals is used as a sacrificialsupport layer during polishing. The second part herein describesembodiments that provide modifications to the electroforming process,specifically: (a) the use of Nickel-Iron (80:20) as a structuralcomponent; (b) modifications to the electroplating bath for generatinglow-stress electroforms; and (c) modifications to the sequence of stepsfollowed during electroforming for fabricating a uniform electroform.Also disclosed in the second part herein is the use of thermal treatmentto the photoresist mold to generate an array of microlens pattern on themaster mold. The second part herein also discloses a simplifiedfabrication process for manufacturing an array of microlenses on themaster mold using electroforming techniques. Finally, a combinedfabrication process that uses the electroforming approach after thetwo-step electroplating and planarization (or polishing) technique isdisclosed for the fabrication of robust master molds suitable for highvolume productions.

Definitions

The process of “Microfabrication” as described herein relates to theprocess used for manufacture of micrometer sized features on a varietyof substrates using standard microfabrication techniques as understoodwidely by those skilled in this art. The process of microfabricationtypically involves a combination of processes such as photolithography,wet etching, dry etching, electroplating, laser ablation, chemicaldeposition, plasma deposition, surface modification, injection molding,hot embossing, thermoplastic fusion bonding, low temperature bondingusing adhesives and other processes commonly used for manufacture ofMEMS (microelectromechanical systems) or semiconductor devices.“Microfabricated” or “microfabricated devices” as referred to hereinrefers to the patterns or devices manufactured using themicrofabrication technology.

The term “BioMEMS” as used herein describes devices fabricated usingMEMS techniques specifically applied towards biochemical applications.Such applications may include detection, sample preparation,purification, isolation etc. and are generally well know to thoseskilled in the art. One such technique that is commonly used in BioMEMSapplications is that of “Capillary Electrophoresis” (CE). CE refers tothe process wherein an electrical field is applied across a liquidcolumn leading to the separation of its constituents based on theirmass/charge ratio. The term “CE Chips” refers to microfluidic BioMEMSdevices specifically used for CE applications.

The term “Optical MEMS” as used herein describes devices fabricatedusing MEMS techniques specifically applied towards optical applications.The term “MOEMS”, which is an abbreviation for“Micro-Opto-Electro-Mechanical-Systems”, is also used interchangeablywith “Optical MEMS” herein.

The term “chip”, “microchip”, or “microfluidic chip” as used hereinmeans a microfluidic device generally containing a multitude ofmicrochannels and chambers that may or may not be interconnected witheach another. Typically, such biochips include a multitude of active orpassive components such as microchannels, microvalves, micropumps,biosensors, ports, flow conduits, filters, fluidic interconnections,electrical interconnects, microelectrodes, and related control systems.More specifically the term “biochip” is used to define a chip that isused for detection of biochemically relevant parameters from a liquid orgaseous sample. The microfluidic system of the biochip regulates themotion of the liquids or gases on the biochip and generally providesflow control with the aim of interaction with the analytical components,such as biosensors, for analysis of the required parameter.

The term “microstructure” as used herein, describes a structure createdusing well-known microfabrication processes wherein at least one of thedimensions of the microstructure ranges from 1 μm to 1000 μm. In thecase of microfluidic devices, the microstructures may be referred to as“microchannels” or simply “channels” whereas for Optical MEMS devices,the microstructures may be referred to as “microlens” or “microlensarray”. It is to be understood that “microstructures” is a generic termwhereas more specific terms are used in contexts where themicrostructures are used for a specific application. Microstructures aregenerally characterized by their “aspect ratio”, which used hereindescribes the ratio of the height to width of the microstructure. Theterm “surface roughness” refers to the root mean square (rms) value ofsurface irregularity for the surface of the microstructure which extendsout of the plane of the substrate and is parallel to the plane of thesubstrate. The term “cross-section” as used herein follows the commonlyaccepted meaning, specifically the area created by a plane cuttingthrough the microstructure.

The term “microchannel” as used herein refers to a groove or pluralityof grooves created on a suitable substrate with at least one of thedimensions of the groove being in the micrometer range. Microchannelscan have widths, lengths, and/or depths ranging from 1 μm to 1000 μm. Itshould be noted that the terms “channel” and “microchannel” are usedinterchangeably in this description. Microchannels can be used asstand-alone units or in conjunction with other microchannels to form anetwork of channels with a plurality of flow paths and intersections.

The term “microfluidic” generally refers to the use of microchannels fortransport of liquids or gases. The microfluidic system consists of amultitude of microchannels forming a network and associated flow controlcomponents such as pumps, valves and filters. Microfluidic systems areideally suited for controlling minute volumes of liquids or gases.Typically, microfluidic systems can be designed to handle fluid volumesranging from picoliter to milliliter ranges.

The term “microlens” as used herein refers to a physical configurationon the substrate that can be used for focusing or diverging an incidentbeam of light, and where at least one dimension of the microlens rangesfrom 1 μm to 1000 μm. The term “microlens array” is used herein todescribe a plurality of microlenses wherein at least 2 microlenses arefabricated in close proximity of each other. The microlens array may besymmetric (i.e. a 2×2 or a 4×4 etc. arrangement of microlenses) orasymmetric (i.e. a 2×1 or a 5×2 etc. arrangement of microlenses). Theterm “Plano-Convex” describes a lens structure wherein one side of thelens is flat and the other side has a convex structure.

The term “substrate” as used herein refers to the structural componentused for fabrication of the micrometer sized features usingmicrofabrication techniques. A wide variety of substrate materials arecommonly used for microfabrication including, but not limited to;silicon, glass, polymers, plastics, ceramics to name a few. Thesubstrate material may be transparent or opaque, dimensionally rigid,semi-rigid or flexible, as per the application they are used for.Generally, microfluidic devices consist of at least two substrate layerswhere one of the faces of one substrate layer contains the microchannelsand one face of the second substrate is used to seal the microchannels.The terms “substrate” and “layer” are used interchangeably in thisdescription. Also, the terms “plastics” and “polymers” are usedinterchangeably. It is to be understood that the terms “plastics” or“polymers” encompass thermoplastics (material which deform whenpressurized at elevated temperatures), thermosets (materials which “set”or attain final shape at elevated temperatures) as well as two-partpolymers (that are mixed for curing). The choice of the plasticsubstrate is dictated by the application area of the MEMS device (e.g.for BioMEMS application; biocompatibility is an important criteria;whereas optical transmission properties may be of greater importance forMOEMS applications) and not limited to a particular material or even aset of materials.

The term “dummy substrate” as used herein refers to a substrate that isused in the fabrication process and is selectively destroyed as a partof the fabrication process such that it does not constitute a part ofthe final product.

The term “UV-LIGA” describes a photolithography process modeled on the“LIGA” fabrication approach. LIGA refers to the microfabrication processfor creating microstructures with high aspect ratio using synchrotronradiation and thick photoresists (ranging in film thickness from 1 μm to5 mm). The LIGA process is used to form a template that can be useddirectly or further processed using techniques such as electroplating tocreate the microfluidic template. UV-LIGA uses modified photoresiststhat can be spin coated in thicknesses of 1 μm to 1 mm and are sensitiveto UV radiation. UV radiation sources are commonly used inmicrofabrication facilities and hence UV-LIGA offers a lower costalternative to LIGA for fabrication of high aspect ratiomicrostructures.

The term “current crowding” as used herein refers to the phenomenonwherein the current flux is concentrated in certain areas of thesubstrate during the electroplating process. Current crowding in turnresults in non-uniform electroplating rates.

The term “reflow” as used herein, refers to the process where athermoplastic material (such as photoresist) is heated beyond a criticaltemperature at which stage it starts changing to a liquid phase. Surfacetension forces will then alter the shape of the molten thermoplasticmaterial to minimize the free surface energy.

The term “sacrificial” as used herein refers to a component used duringthe fabrication process which is only present in intermediate steps ofthe fabrication process and is completely destroyed in the process suchthat it does not form a part of the fabricated device.

The term “chemical mechanical polishing (CMP)” as used herein describesa process wherein a reasonably flat substrate is polished on one or bothsides to achieve parallelism between two opposite surfaces as well asdesired surface uniformity on one or both surfaces. This process is alsoreferred to as “planarization” and the two terms are usedinterchangeably in this description.

The term “electroforming” as used herein carries the same meaning as theconventionally accepted meaning to those skilled in this art.Specifically, it is “An electrochemical process of master moldfabrication using an electrolyte, an anode to supply the metal, and acontrol of the electrical current and of the deposition of metal onto asuitable mold, which is fabricated on a dummy substrate.”

The term “master mold” as used herein refers to a replication template,typically manufactured on a metallic or Silicon substrate. Specificallyherein, “master mold” refers to a metallic mold wherein themicrostructures are either an integral part of the master mold or aredeposited on one of the surfaces of the master mold. The features of themaster mold are fabricated using the UV-LIGA and other microfabricationprocesses. The microstructures created on the master mold may be of thesame material as the master mold substrate e.g. Nickel microstructureson a Nickel substrate or may be a dissimilar material e.g. photoresiston a Silicon surface. The master mold is typically used for creatingpatterns on a polymer substrate using techniques such as hot embossing,injection molding, and casting.

The term “bonding” as used herein refers to the process of joining atleast two substrates, at least one of which has microfabricatedstructures, e.g. a microchannel, on its surface to form a robust bondbetween the two substrates such that any liquid introduced in themicrochannel is confined within the channel structure. A variety oftechniques can be used to bond the two substrate including thermoplasticfusion bonding, liquid adhesive assisted bonding, use of interfacialtape layers, etc. Specifically, in this description, the terms “bonding”and “thermoplastic fusion bonding” are used interchangeably.Thermoplastic fusion bonding involves heating the two substrates to bejoined to their glass transition temperature and applying pressure onthe two substrates to force them into intimate contact and cause bondformation. Another bonding process, namely the use of UV-adhesiveassisted low temperature bonding, is also described herein and isspecifically and completely referred to in all occurrences.

The intent of defining the terms stated above, is to clarify their usein this description and does not explicitly or implicitly limit theapplication of embodiments of the present invention by modifications orvariations in perception of said definitions.

Two-step electroplating and planarization technique for master moldfabrication using UV-LIGA process:

FIG. 1 a shows the basic concept of a replaceable master mold diskwithin the mold block. As shown in FIG. 1 a, the injection mold blockconsists of two halves A 102 and B 103. Mold Block A 102, houses themaster mold whereas Mold block B 103, has two cavities opposite to themaster molds and the depth of these cavities defines the thickness ofthe injection molded plastic part. The replaceable master mold 100contains microfabricated features 101 and is mounted on the A block 102.The replaceable mold disks are mounted on mounting cylinders 105 asshown in FIG. 1 b and the mounting cylinder plus mold disk assembly ispositioned within the mold block half A 102 (note that the schematicsketch shows only one mold disk whereas the actual mold block cancontain two master molds at the same time). After inserting the mountingcylinders and master mold, the two mold blocks are assembled togetherand mounted in an injection molding machine (not shown). The injectionmolding machine contains a hopper to store plastic pellets from whichthey are fed into an extrusion screw, which is maintained at an elevatedtemperature. The plastic pellets are melted in the screw and forced outas molten plastic through a nozzle which connects to the inlet in moldblock B 103. The molten plastic is then injected through an inlet 106 inthe B block 103 shown in FIG. 1 b. The molten plastic fills up thecavity in mold block B 103. One end of this cavity is exposed to themaster mold and the molten plastic will replicate the shape of themaster mold. Upon cooling, the plastic part will solidify and issubsequently ejected using ejector pins 104. It is clear from thisdescription that the microstructures in the plastic part will be anegative image of those on the mold master and using appropriateinjection molding conditions very high fidelity reproductions can beachieved. Hence, it is critical that the master mold microstructures arevery precisely and accurately defined.

FIGS. 2 a-2 g show the conventional UV-LIGA process that is commonlyused to make the master mold. As shown in FIG. 2 a, initiallyphotoresist 210 is spin-coated onto a (typically) metallic substrate200. Most commonly a Nickel substrate is used because of its excellentmechanical properties as well as the ease of subsequent electroplatingof Nickel microstructures. The negative photoresist 210, is exposed to aUV-radiation 213 through opening 215 in a photomask 214. Next, theexposed photoresist is developed in a suitable developer to retainphotoresist in the exposed areas 216, and washed out to expose theunderlying metal layer in the unexposed areas 218, as shown in FIG. 2 cand FIG. 2 d. Note that the location of the resist areas and cavitiescan be reversed by using a positive photoresist, while using the samephotomask. Following development, Nickel 219 (or the same metal as theunderlying substrate) is electroplated within the cavities of thephotoresist mold. After this step, the photoresist mold is dissolvedusing a suitable solvent to leave behind free standing microstructures201 as shown in FIG. 2 f.

Although this process or minor variations thereof is very commonly used,it suffers from a couple of serious drawbacks. As shown in the magnifiedview 212 of FIG. 2 b, the spin-coated photoresist does not coat theentire substrate uniformly. Specifically, because of difference incentripetal force experienced during spin-coating and due to surfacetension forces at the edge of the wafer, the photoresist tends to form aso-called “edge-bead” along the circumference of the substrate. Thisproblem can be partly circumvented by using alternate coating techniquessuch as spray-coating or dip-coating but these techniques are not widelyaccepted in the art.

Another and more serious problem is due to the phenomenon of currentcrowding. In the electroplating setup, the anode is (typically)approximately equal in area and dimensions to the substrate (which actsas the cathode in the electrochemical reactions involved inelectroplating). The anode and the substrate are maintained at aspecific distance from each another and a constant (or pulsed) currentis applied across this arrangement. Because of the difference in widths,the current flux tends to be more concentrated at the narrower featuresleading to a faster electroplating rate and higher microstructures 223when compared to microstructures with larger widths 224. Furthermore,even along the width of the microstructures, due to current fluxconcentration at the edges, the edges of the microstructures 225 tend tobe taller than the middle sections 226. These problems are well known inthe art and are tolerated as there has been no solution proposed whichcan effectively address these problems. For example, if a pulsed currentis used wherein an asymmetrical square wave current with longer durationin the positive cycle is used, it has been demonstrated that thisproblem is minimized yet it is not fully addressed and variations,though of a lesser magnitude, are still observed.

The non-uniform microstructure dimensions can be a significant problemin microfluidics applications. In this case, the microchannels need tobe of precise dimensions to accurately govern the flow characteristics,and even small variations in the dimensions can lead to substantialchanges in flow characteristics. For example, it has been observed inour experiments that a 20 μm wide channel is electroplated almost twiceas fast as a 500 μm wide channel leading to significant variations inthe volumes that can be accommodated in these channels. Furthermore,this discrepancy is not even predictable and can be severely affected bythe overall layout, density and physical proximity of microchannels ofdifferent widths.

FIGS. 3 a-3 j show the sequence of events for the two-stepelectroplating and planarization method, in accordance with anembodiment of the present invention, which addresses most of theproblems listed above. As is readily obvious, steps shown in FIGS. 3 a,3 b and 3 c are similar to those described above. However, as shown inFIG. 3 d, the microstructures 319 are electroplated beyond the thicknessof the photoresist so that each microstructure extends beyond 320 thephotoresist mold. Following this step, the photoresist is dissolved in asuitable solvent as shown in FIG. 3 e. Also, some sections of thesubstrate are blocked using an electrical non-conductive layer 321 toprevent electroplating in these areas. Then a second sacrificial metallayer 330 is electroplated on the substrate such that this metal layerextends well beyond the thickness of the electroplated microstructuresand effectively covers all the microstructures. In an embodiment of thepresent invention, the materials involved are a Nickel substrate,electroplated Nickel microstructures, and a sacrificial Copper layer.These are by no means a unique combination and indeed any combinationsof metallic substrates and sacrificial layer may be used with equalsuccess. The only criterion for choosing the metal pair is that thesacrificial metal layer can be etched without affecting the substrateand electroplated microstructures. Secondary factors such as costprohibit the use of obviously precious metals such as Gold thoughtechnically that may also be used since it can be selectively etchedagainst (in this case) the Nickel patterns.

Following electroplating of the sacrificial metal layer, the substratewafer is flipped over and polished using CMP techniques. Initially thesacrificial metal layer is polished out using a coarse grit, until thefirst microstructures (which would be the narrowest and hence thetallest) start showing through. Following this the CMP is done using afine grit to ensure slow removal and low surface roughness on the topsurface of the microstructures. An intermediate step in the polishingprocess is when all the microstructures can be seen without anysacrificial metal on top. At this stage all the microstructures are ofexactly the same height and furthermore, have uniform height across thewidth of the pattern. The height 331 of the polished microstructures andsacrificial metal layer is checked periodically as shown in FIG. 3 g andlapping is continued until the desired height is obtained. As is readilyapparent from the drawing, the polishing step could also have beenstarted after step 4 (FIG. 3 d) in the sequence. However, the relativelysoft photoresist material cannot provide enough structural support tohigh aspect ratio microstructures during the polishing process. A veryhigh shear force is exerted on the microstructures during polishing andif they are not properly supported in the sides, they can easily peeloff the substrate. The sacrificial metal layer is used precisely forthis reason to provide the strong support required.

Following the polishing step, the sacrificial metal layer is selectivelyetched using an etchant that does not affect the substrate orelectroplated metal. At this stage the master mold 350, containsmicrostructures 301 which are of extremely uniform height across theentire mold surface. Furthermore, as shown in the magnified view 322, ofFIG. 3 i the patterns are not only of exactly the same height but alsoare perfectly uniform across the entire width of the patterns. FIG. 3 jshows the replicated plastic part 399, using the microfabricated mastermold 350.

The surface roughness on the top of the microstructures is directlygoverned by the grit size of the polishing compound use during the finalsteps of the polishing process. By choosing an appropriate grit size(which are available down to nm size), a very uniform surface withultra-low rms roughness value can be achieved. In the preferredembodiment, the final grit size used for polishing is a 150 nm gritwhich yields rms surface roughness of <50 nm across all themicrostructures. This can obviously be changed easily by using differentgrit sizes.

The two-step electroplating described above is well suited for certainMEMS applications such as microfluidics. Without intent of limiting thescope of embodiments of the present invention, it is anticipated thatthis technique could primarily benefit the development of high accuracymaster molds for microfluidic plastic devices. However, this techniqueis not well suited to the fabrication of microstructures withnon-rectangular cross sectional areas. The electroforming processdescribed in the next two sub-sections is more appropriate for creatingfeatures with different cross-sectional profiles. Another issue thatneeds to be addressed is the longevity and durability of the mastermold. Since, in the two-step electroplating process (or in the commonlyaccepted UV-LIGA process) the microstructures are deposited onto asubstrate, there are inherent problems with the adhesion of themicrostructures to the base substrate layer. In accordance with anembodiment of the present invention described above, Nickelmicrostructures are deposited onto a Nickel substrate, or moregenerically stating, the microstructures are commonly the same materialas the substrate to ensure that there is good adhesion and matching ofthe compressive or tensile stress between the two. Nevertheless, themicrostructures are still “foreign” to the substrate and after repeatedinjection molding cycles, the microstructures eventually start peelingoff because of the high shear rate exerted on them during the injectionmolding process. In the electroforming process, the microstructures forman integral part of the substrate since the base substrate layer is“grown” on top of the microstructures hence, in this case adhesionfailure is not an issue of concern. Hence, for applications where it isanticipated that a very large volume of plastic parts will be replicated(e.g. in high volume manufacturing scenarios) the electroforming processmay be more suitable than the UV-LIGA or two-step electroplatingprocess. The process for developing master molds with the same highaccuracy is described later in this description.

Master Mold Fabrication Using a Modified Electroforming Process:

As explained previously, the electroforming process is particularly wellsuited for fabricating features with non-rectangular cross-sectionalprofiles. One such process is illustrated schematically in FIGS. 4 a-4i. Again, as in previous cases, a layer of photoresist 410 is depositedonto a dummy substrate 400. In accordance with an embodiment of thepresent invention, the dummy substrate is a 2 mm thick Silicon wafer. Ofcourse, as is obvious to those skilled in the art, a number of differentdummy substrate material may be used such as, but not limited to, Glass,Ceramics, rigid plastics, or even highly polished metallic substrates.After depositing the photoresist, it is exposed to UV-radiation 413through openings 415 in a photomask 414 as shown in FIG. 4 b. Followingthis, the exposed photoresist is developed with a suitable developersolution and the unexposed regions (in the case of a negative resist)are washed away.

At this stage, a free standing photoresist pattern is formed on thedummy substrate as shown in FIG. 4 c. After this, the substrate plusphotoresist patterns are heated to the so-called glass transitiontemperature of the photoresist. Most photoresist consist of a polymericor epoxy backbone with light sensitive chemicals added thereafter. Thephotosensitive material confers the patterning ability. However, most ofthe polymeric or epoxy bases used for photoresist are thermoplastic innature wherein application of heat (to cured or hardened photoresist)causes it to gradually transition from a solid to a gel to a liquid.Surface tension forces come into effect during the transition and modifythe shape of the photoresist patterns. The surface tension forces alwaystry to minimize the surface area in order to minimize the free surfaceenergy. It is well known that a sphere has the lowest surface area for agiven volume and the surface tension forces try to shape the moltenresist into a spherical shape. However, surface tension forces are alsoactive at the interface between the molten resist and the substratematerial which prevent formation of the spherical shape. For anyliquid-solid pair there exists a contact angle depending on the varioussurface tension forces. As shown in FIG. 4 d, upon heating, therectangular cross-section is initially rounded at the edges then thevertical sidewalls become more sloped and eventually in the ideal case,the molten photoresist will achieve a shape resembling a section of asphere. The final shape of the molten photoresist depends upon thetemperature, the duration of exposure to the elevated temperature, thevolume of the photoresist, the initial shape of the photoresist, thecomposition of the photoresist itself, the contact area per unit volumebetween the photoresist and the substrate and the contact angle betweenthe molten resist and the substrate. Varying any one of these conditionscan lead to a different shape. The concept of melting photoresist toachieve non rectangular cross-sectional areas has never been applied togenerating a quasi semi-spherical shape before. In accordance with anembodiment of the present invention, the photoresist is heated toapproximately 10° C. higher than the glass transition temperature andthe temperature is maintained for an extended period of time (rangingfrom a few minutes to more than 2 hours) to allow the photoresist toreflow completely. Thus, in this case the primary governing factorsdetermining the final shape are very stable and repeatable for a givenphotoresist/substrate combination. Of course, any of the intermediateshapes shown in FIG. 4 d can also be achieved.

As shown in FIG. 4 e, depending on the initial shape (narrow or wide)the photoresist will reflow to quasi semi-spherical shapes withdifferent radii of curvature 417, 427 and 437. As is readily apparent,this is of great significance for MOEMS applications, wherein eachdifferent radius of curvature can generate a lens with a different focalpoint.

Following the photoresist reflow step, the dummy substrate is completelycooled down (to approximately room temperature or ˜25° C.) and the topand sides are coated with a metallic seed layer 433. One of the novelideas exercised in this invention is that the SIDES as well as the “top”surface of the substrate is coated with the seed layer whereas the backside does not have a seed layer coating. The reasons for this will beapparent from further disclosures later in this section. The dummysubstrate is then immersed in an electroplating bath for deposition ofthe actual substrate material. As explained previously, normally Nickelis used as a substrate material owing to its suitable mechanicalproperties. For this particular application, Nickel is not a suitablesubstrate material since electroplated Nickel films develop very highstress and are likely to peel off the dummy substrate.

In accordance with an embodiment of the present invention, we disclosethe use of Nickel-Iron (80:20) as a substrate material. Formerly,Nickel-Iron (80:20) has been extensively used as a material withexcellent magnetic properties. However, in accordance with an embodimentof the present invention, we have used Nickel-Iron (80:20) for the firsttime as a structural component of the master mold. ElectroplatedNickel-Iron (80:20) has considerably lower stress than Nickel therebyallowing for much thicker electroplated films. Indeed the exampleslisted in the Background section herein all report electroformed filmwith thicknesses ranging from 200 μm to 500 μM. The only cases whichreport higher thicknesses use electroforming on large scale substratesand are not related to microfabrication processes. Furthermore, we havealso modified the electroplating bath composition used for Nickel-Iron(80:20) electroplating. Normally, a Nickel-Iron (80:20) electroplatingbath contains 3 gm/l of saccharin to reduce the tensile stress on theelectroformed film. In accordance with an embodiment of the presentinvention, we have used 5 gm/l of saccharin which is an optimumconcentration to minimize the tensile stress for creating thickelectroforms. The composition of the Nickel-Iron (80:20) electroplatingbath (total volume 8 liters) is shown in Table 1. TABLE 1 Compoundconcentration (g/l) Nickel sulphate 200 Iron sulphate 8 Nickel chloride5 Boric Acid 25 Saccharin 5

In order to further reduce the probability of the electroform peelingoff the dummy substrate, electroplating is also done of the sides of thesubstrate as shown in FIG. 4 f. From our experiments we have concludedthat without electroplating on the sides of the substrate it is almostimpossible to create a electroform of substantial thickness since evenwith the reduced stress of Nickel-Iron (80:20) and the extra Saccharin,the electroform will still peel away if it is not clamped at the sides.The growth of the electroform on the sides of the dummy wafer anchorsthe electroform firmly to the dummy substrate and allows for higherthicknesses.

It is well known in the art that a number of parameters such as pH,temperature and current density affect the electroplated Nickel Ironalloy. Lower current densities lead to decreased current efficiency thatleads to secondary cathode reactions like evolution of hydrogen thatsignificantly increases stress, whereas higher current densities lead tovery fast electroplating, which is usually under massive tensile stress.Our experiments have demonstrated that an optimum current density of 10mA/cm² leads to increased current efficiency and low stress deposition.During electroplating the pH was maintained between 2.9-3.1. The pHincreases with plating duration and is controlled via the addition ofdilute sulfuric acid. The plating rate was ˜9-10 micrometer/hour. Theelectroplating was carried out at room temperature to avoid thermallyinduced stress. The electroplating was carried out for a period ofaround 160 hours to obtain a plated thickness of ˜1.6 mm. It isimpossible to avoid the roughness 432 at the back end of the electroform430.

Following this step, the back of the electroform is polished to make itplanar as shown in FIG. 4 g. By visual inspection, no curvature orbending due to stress was observed during actual fabrication processes.The thick silicon dummy substrate 400 is then removed from theelectroplated Nickel Iron layer by chemical-mechanical polishing fromthe dummy substrate side (note that the dummy substrate can also beremoved by using a selective etchant that does not affect theelectroform). Then the seed layer is etched out using suitable selectivemetal etchants. Finally, the electroform is machined to trim the edgesand create a finished circular substrate 450 with concave depression asshown in FIG. 4 h. FIG. 4 i shows the replicated plastic part clearlyillustrating that each concave depression in the master-mold results ina Plano-Convex lens shape of the plastic substrate. Since this processis based on microlithography techniques it is possible to create andarray of microlenses with equal ease.

If the intent of the fabrication process is specifically the developmentof microlens arrays, a highly simplified yet much more versatileapproach can also be used as illustrated in FIGS. 5 a-5 f. In this case,the mold material 507, which can be any polymer composition that iseasily cured at room temperature, or at elevated temperatures or byexposure to (say) UV light etc., is directly deposited onto the dummysubstrate in the form of droplets 508. Conventional micro-dispensers 509capable of accurate dispensing in the microliter-nanoliter range arewell known in the art and are in fact commercially available.Furthermore, such dispensing systems also incorporate a high precisionX-Y stage such that the liquid can be dispensed at precise locations onthe substrate over the entire area of the substrate. In addition tosimplicity, another significant advantage of this technique is the widechoice of materials that are available for this application. Since themold material in this case does not have to be a photosensitive materialsuch as photoresist, a huge variety of materials can be used for thisapplication. The only constraints in the choice of the mold material arethat (a) it should exhibit good adhesion to the dummy substrate and (b)it can selectively be etched against the electroform material.Furthermore, in order to control the spread of the dispensed liquid ontothe substrate, the surface energy (and hence that contact angle) of thesubstrate can be modified by using a wide variety of techniques wellknown in the art.

Following deposition of the droplets onto the dummy substrate, thesurface tension forces will govern the final shape of the droplet asexplained previously. All the factors listed previously also apply herewith the exception of the high temperature and the duration of exposureto elevated temperatures since these are not required in this case. Thequasi semi-spherical droplets 507 will then be cured to a solid in theirfinal positions as shown in FIG. 5 b. Since the lens array mold iscreated by direct deposition of the mold material, it is alsoconceivable that a variety of materials be used for creating lenses withslightly different shapes. For example, if a low viscosity and anotherhigh viscosity mold material are used on the same substrate, the lowviscosity material would generate lens shapes with very long focaldistance (due to larger radius of curvature) whereas, the high viscositymaterial would create lens shapes with shorter focal lengths (due tosmaller radius of curvature). Using this approach, it is possible tocreate lenses with different focal lengths as part of the same array orsuch that each lens array has identical lenses but that each arrayvaries from another one on the same substrate.

After this step, the dummy substrate and the mold pattern areelectroplated to create the electroform 550 and, subsequently, thereplicated plastic device 599 with the array of Plano-Convex lenses, asshown in FIGS. 5 e-5 f, in a process similar to the one explainedpreviously.

In yet another embodiment, the two processes described above can becombined to fabricate a master mold with the high accuracy and precisionof the two-step electroplating process as well as the robustcharacteristics of the electroforming process. This process isillustrated schematically in FIGS. 6 a-6 k.

The initial sequence of events is exactly the same as the two-stepelectroplating process wherein photoresist 610 is coated onto a dummysubstrate 600, then exposed to UV radiation 613 through and appropriatephotomask 614 to create a photoresist mold pattern 616 and 618. Inaccordance with an embodiment of the present invention, Nickel is thenelectroplated 619 beyond the thickness of the photoresist mold 620.Then, the photoresist mold is removed and copper 630 is electroplated tocover the Nickel microstructures. Then the plated patterns areplanarized and polished to the desired height 631.

Following this step, the NICKEL is now selectively etched out (insteadof etching out the copper as is done in the previous approach) to leavea free standing copper microstructure pattern 611 as shown in FIG. 6 h.Then a metal seed layer is deposited in the microstructures, the exposedareas on the top of the dummy substrates as well as the sides of thedummy substrate. Following this, the assembly is immersed in aNickel-Iron (80:20) plating bath and an electroplating process similarto the one described in the electroforming approach is used to create aNickel-Iron (80:20) electroform. Then, the dummy substrate is removed byCMP and the copper microstructures are selectively etched out. Then, theelectroform is machined to the desired dimensions to achieve the finalmaster mold 650 shown in FIG. 6 j and subsequently used for plasticreplication to create patterns as shown in FIG. 6 k.

This approach combines the advantages of the two techniques disclosedpreviously. Since, the two-step electroplating process and planarizationapproach is used to define the copper patterns on the dummy substrate,all the microstructures are of uniform height, exhibit uniform heightacross their widths, and have very low surface roughness. Since themaster mold is actually an electroform, the microstructures on themaster mold are now an integral part of the mold and hence are notsubject to peeling and consequently can be used for many morereplication cycles.

It will be readily obvious to those skilled in the art that the choiceof dummy substrate, electroplated microstructure material, sacrificialsupport layer material and electroforming material is not unique and canbe easily extended to include other materials. Furthermore, theapplication areas listed for the above disclosures are by no meanscomplete and the techniques disclosed in accordance with variousembodiments of the present invention can be readily applied to a broaderspectrum of applications where high accuracy and precision are importantaspects for the master mold design. Such modifications do not departfrom the essential novelty of the present invention and are herebyincorporated within the scope of the present invention.

The aforementioned fabrication processes offer numerous advantages aswell as fabrication options for MEMS devices, a few of which areenumerated hereafter.

An advantage of certain embodiments of the present invention is theability to fabricate extremely uniform microstructures (in terms ofheight) across the entire area of a large master mold.

Another advantage of certain embodiments of the present invention is theability to fabricate a master mold containing microstructures withextremely uniform height across the entire width of the microstructure,irrespective of the width itself.

Yet another advantage of certain embodiments of the present invention isthe ability to fabricate a master mold containing microstructures withultra-low surface roughness on the top surface of the microstructures.

Yet another advantage of certain embodiments of the present invention isthe ability to fabricate a master mold with high aspect ratiomicrostructures that incorporate the advantage listed above.

Yet another advantage of certain embodiments of the present invention isthe ability to fabricate a thick (greater than 1 mm thickness) mastermold using modifications to the electroforming techniques andspecifically tailoring it towards MEMS based master mold development.

Yet another advantage of certain embodiments of the present invention isthe ability to create electroforms with low tensile stress.

Yet another benefit of certain embodiments of the present invention isthe ability to use existing electroplating setups and other standardmicrofabrication techniques without the need to develop specializedequipment.

Yet another advantage of certain embodiments of the present invention isthe ability to create robust master molds wherein the microstructuresare an integral part of the master mold thereby allowing use of themaster mold for high volume production runs of plastic replication.

Yet another advantage of certain embodiments of the present invention isthe ability to manufacture precise microlens arrays with a wide range offocal lengths using a highly simple fabrication process.

Yet another advantage of certain embodiments of the present invention isthe quick turn-around time offered by the use of the replaceable molddisk technology with the high accuracy, robust master molds therebyavoiding the necessity of having to change the entire mold block for anew design,

Yet another advantage of certain embodiments of the present invention isthe reduction in manufacturing costs achieved by eliminating the need toreplace the entire mold block for each new design.

Yet another advantage of certain embodiments of the present invention isthe ability to develop a wide variety of cross sectional profilesincluding square/rectangular, rounded trapezoidal, quasi-semi circular,and semi-circular.

Yet another advantage of certain embodiments of the present invention isthe wide choice of material that can be employed for use as substrate,dummy substrate, sacrificial support layer and mold material.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A method to generate a master mold used for micro-fabricating plasticsubstrates, said method comprising: depositing a micro-structure patternof photo-resist onto a substrate; over-plating a first metal onto saidpattern to generate a plurality of metal micro-structures; removing saidpattern of photo-resist from said substrate; and over-plating a secondmetal onto said substrate and said plurality of metal micro-structures.2. The method of claim 1 further comprising lapping said second metaland said plurality of metal micro-structures to create a planar surface.3. The method of claim 2 further comprising removing said second metalfrom said substrate and said plurality of metal micro-structures,leaving said master mold comprising said substrate and said plurality ofmetal micro-structures.
 4. The method of claim 1 wherein said substratecomprises nickel.
 5. The method of claim 1 wherein said first metalcomprises nickel.
 6. The method of claim 1 wherein said first metalcomprises an alloy of nickel and iron.
 7. The method of claim 1 whereinsaid second metal comprises copper.
 8. The method of claim 1 whereinsaid depositing said micro-structure pattern of said photo-resistcomprises performing a photolithography process.
 9. The method of claim1 wherein said over-plating said first metal comprises performing anelectroplating process such that said first metal is electroplated pasta height of said photo-resist.
 10. The method of claim 1 wherein saidremoving said pattern of said photo-resist comprises performing astripping process using a remover solution.
 11. The method of claim 1wherein said over-plating said second metal comprises performing anelectroplating process such that said second metal is electroplated pasta height of said plurality of metal micro-structures.
 12. The method ofclaim 2 wherein said lapping comprises performing a chemical-mechanicalpolishing process.
 13. The method of claim 3 wherein said removing saidsecond metal comprises performing an etching process.
 14. A method togenerate a master mold used for micro-fabricating plastic substrates,said method comprising: generating a micro-structure pattern on asubstrate; heating said micro-structure pattern to form a pattern of atleast one quasi-semi-spherical feature on said substrate; coating saidsubstrate and said pattern of at least one quasi-semi-spherical featurewith a seed layer; and depositing a metal layer onto said seed layer.15. The method of claim 14 further comprising polishing a back of saidmetal layer to form a planar surface.
 16. The method of claim 15 furthercomprising removing said substrate from said metal layer, said seedlayer, and said at least one quasi-semi-spherical feature.
 17. Themethod of claim 16 further comprising removing said seed layer to leaveonly said polished metal layer as said master mold.
 18. The method ofclaim 14 wherein generating said micro-structure pattern is accomplishedusing a photo-resist and photolithography techniques.
 19. The method ofclaim 14 wherein said micro-structure pattern comprises photoresist. 20.The method of claim 14 wherein said substrate comprises one of silicon,glass, ceramic, plastic, and metal.
 21. The method of claim 14 whereinsaid metal layer comprises an alloy of Nickel and Iron.
 22. The methodof claim 14 wherein said step of depositing a metal layer comprises anelectroplating step.
 23. The method of claim 16 wherein said substrateis removed using a chemical-mechanical polishing technique.
 24. Themethod of claim 17 wherein said seed layer is removed using an etchingtechnique.
 25. A method to generate a master mold used formicro-fabricating plastic substrates, said method comprising: generatinga micro-structure pattern of at least one quasi-semi-spherical featureon said substrate by dispensing droplets of a polymer material onto saidsubstrate; allowing said droplets of polymer material to cure; coatingsaid substrate and said pattern of at least one quasi-semi-sphericalfeature with a seed layer; and depositing a metal layer onto said seedlayer.
 26. The method of claim 25 further comprising polishing a back ofsaid metal layer to form a planar surface.
 27. The method of claim 26further comprising removing said substrate from said metal layer, saidseed layer, and said at least one quasi-semi-spherical feature.
 28. Themethod of claim 27 further comprising removing said seed layer to leaveonly said polished metal layer as said master mold.